1. Field of the Invention
The present invention relates to a display apparatus using light emitting devices, such as organic EL (electro-luminescence) devices, and more particularly to a drive method of the light emitting devices.
2. Related Background Art
In a flat display panel including organic EL devices or the like, pixels arranged in a plurality of rows and in a plurality of columns are commonly connected to a scanning line on a row basis and to a data line on a column basis, and each scanning line is selected by a row scanning circuit. At the same time, a column scanning circuit applies a predetermined display signal to each data line to make the pixels on the selected row perform a predetermined display. Such matrix driving is generally performed. As to the matrix driving, U.S. Pat. No. 6,373,454 discloses an EL display panel using active matrix driving.
A display panel including the organic EL devices can adjust the emission intensity of each pixel by controlling the current flowing through the organic EL device in the pixel.
FIG. 12 is a block diagram illustrating the schematic configuration of an active matrix type display panel of related art. In this figure, the display panel includes a current setting circuit 201, a scanning line drive circuit 202, and pixel circuits 203 arranged in a matrix.
FIG. 13 illustrates a configuration example of a related-art pixel circuit including an organic EL device. The pixel circuit includes scanning lines P1 and P2, and an information line P0. Current data Idata is input from the information line P0 as an information signal. The anode of the organic EL device is connected to the drain terminal of a TFT M4, and the cathode of the organic EL device is connected to the ground potential CGND. Transistors M1, M2, and M4 are P type TFTs, and a transistor M3 is an N type TFT.
The outline of the operation of the pixel circuit is next described. When the current data Idata is input, a high level (hereinafter referred to as an HI level) signal is input into the scanning line P1, and a low level (hereinafter referred to as a LOW level) signal is input to the scanning line P2. The transistors M2 and M3 are turned on, and the transistor M4 is turned off. At this time, because the transistor M4 is not in its conductive state, no current flows through the organic EL device. A voltage corresponding to the current drive performance of the transistor M1 is generated in a capacitor C1 disposed between the gate terminal of the transistor M1 and power potential V1 due to the current data Idata.
When a current is supplied to the organic EL device, a LOW level signal is input into the scanning line P1, and an HI level signal is input into the scanning line P2. At this time, the transistor M4 is turned on, and the transistors M2 and M3 are turned off. Because the transistor M4 is in its conductive state, a current corresponding to the current drive performance of the transistor M1 is supplied to the organic EL device due to the voltage generated in the capacitor C1. The organic EL device emits a light of the brightness according to the supplied current.
By the way, it is difficult to adjust the brightness of the display panel using such organic EL devices. In the case of a liquid crystal display panel, the only thing required for the liquid crystal display panel is to adjust the brightness of the back light thereof, and consequently the liquid crystal display panel can relatively easily adjust the brightness of the display panel without changing the image quality level thereof.
On the other hand, in the case of the organic EL device, because the brightness thereof is controlled by the quantity of a current flowing through each pixel, it is necessary to control the whole display panel in a low current region in order to darken the whole display panel. The deterioration of the image quality of the display panel is caused owing to the use of the low current region, the controllability of which is bad.
Accordingly, there is a technique of controlling the light-emitting period of the organic EL device in order to adjust the brightness of the whole display panel. However, there is a problem of the occurrence of a flicker in the case where a non-luminous time zone occupies 10-30% or more of a period in one field. This flicker appears to have the tendency of becoming easy to observe when a black (non-luminous) band moving on a screen during a non-luminous period is wide.
Moreover, there is a technique of narrowing the black band generated in the non-luminous period in order to cope with both the brightness adjustment and the flicker decreasing. That is, this technique increases the on-and-off rate of light emitting, keeping a rate of an image data writing into pixel circuits fixed (for example, 60 Hz). The technique is disclosed in Japanese Patent Application Laid-Open No. 2006-053236.
By the aforesaid (high frequency on-and-off type) method of increasing the on-and-off rate of light emitting while fixing the rate of the image data writing into the pixel circuits, for example, at 60 Hz, the visibility of a display panel is the same as that of a hold type display panel. In this case, there is a problem of the lowering of the resolution of a moving image as compared with that of an impulse type display panel.
FIGS. 14A and 14B illustrate a display state of a moving image, and an apparent image, of an impulse type display panel, respectively. FIGS. 15A and 15B illustrate a display state of a moving image, and an apparent image, of a hold type display panel, respectively. FIGS. 16A and 16B illustrate a display state of a moving image, and an apparent image, of a high frequency on-and-off type display panel, respectively. FIGS. 14A, 15A, and 16A each illustrate relations between light-emitting states of a moving image, a time axis and views. Their abscissa axes each indicate the positions of pixels, and their ordinate axes each indicate times (fields). The bold line arrows illustrate motions of viewpoints.
FIGS. 14B, 15B, and 16B illustrate apparent images. Their abscissa axes each indicate relative positions of appeared images, and their ordinate axes each indicate brightness. The positions a-j illustrated in FIGS. 14B, 15B, and 16B correspond to the positions a-j illustrated in FIGS. 14A, 15A, and 16A, respectively. It is expressed that the edges of the images are seen more conspicuous as the inclinations of the lines in FIGS. 14B, 15B, and 16B become steeper. Moreover, it is expressed that the edges of the images are seen to be more blurred as the inclinations of the lines in FIGS. 14B, 15B, and 16B become gentler.
As illustrated in FIGS. 14A, 15A, and 16A, it is found that a viewpoint also continuously moves, following the movement of an object when the object moves on a screen. At that time, the apparent brightness that is recognized as visual information is an integrated quantity of the brightness at each position in the field of vision.
Consequently, as shown in FIG. 14B, an edge is seen to be conspicuous on an impulse type display panel, and an edge is seen to be blurred on a hold type or a high frequency on-and-off type display panel as shown in FIG. 15B or 16B, respectively, on the other hand. Consequently, it has been a subject to decrease the flicker with the resolution of a moving image heightened.